How to switch between viewСontrollers using pushViewController?

Question

I have 3 controllers: MainViewController, FirstViewController, SecondViewController.

In SceneDelegate I have installed a rootViewController:

func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        guard let winScene = (scene as? UIWindowScene) else { return }
            window = UIWindow(windowScene: winScene)
            window?.rootViewController = MainViewController()
            window?.makeKeyAndVisible()
    }

How do I make the transition from FirstViewController to SecondViewController using pushViewController()?

FirstViewController:

@objc func buttonAction(sender: UIButton!) {
        print(1)
        let vc = SecondViewController()
        navigationController?.pushViewController(vc, animated: true)
        print(2)
    }

The transition using present() works.

Answer 1

as @Rob said, at first you have to be using navigation controller within your current view controller hierarchy before you can use navigationController?.pushViewController

you have to doing like this:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
   self.window = UIWindow(frame: UIScreen.main.bounds)
   let navController = UINavigationController()
   let mainViewController = MainViewController()
   navController.viewControllers = [mainViewController]
   self.window!.rootViewController = navController
   self.window?.makeKeyAndVisible()
}